Coordination expansion

Because the velocities and stresses are continuous, differentiable functions of the spatial coordinates, expansion in a Taylor series is appropriate across the dimensions of the differential control volume. At the z + dz, r + dr, and 6 + d9 faces the rates of work done on the control volume are... 

In the condensed phase Me3SiF molecules show no intermolecular interactions, while the same germanium derivatives are associated as a dimer due to intermolecular F Ge coordination. According to the tendency for tetrahedral main-group 14 elements to expand the coordination sphere, organotin fluorides show a strong tendency to associate in the solid state and even in triorganotin fluorides the tin atom is five-coordinate A common feature of this class of compounds in the solid state is coordination expansion of the tin atom due to hypervalent interaction, which in turn often results in formation of polymeric materials. 

Because of coordination expansion, most metal alkoxides, other than silicon, are highly reactive toward hydrolysis and condensation. Precipitation occurs as soon as water is added. Therefore their chemical reactivity has to be decreased in order to avoid uncontrolled precipitation. This can be performed via the chemical modification of the molecular precursor prior to hydrolysis. Nucleophilic chemical additives are currently employed in order to stabilize highly reactive metal alkoxides and control the formation of condensed species. 

The sol gel chemistry of sihcon alkoxides is much simpler (see Silicon Inorganic Chemistry)P Si is fourfold coordinated (N = z = 4,) in the precursor as well as in the oxide so that coordination expansion does not occur. The electronegativity of Si is rather high compared to transition metals. Silicon alkoxides are therefore not very sensitive toward hydrolysis. Their reactivity decreases when the size of the alkoxy groups increases. This is mainly due to steric hindrance, which prevents the formation of hypervalent sihcon intermediates (see Hypervalent Compounds). Silicon alkoxides, Si(OR)4, are always monomeric. Heterometallic alkoxides have never been obtained via the reaction of a sihcon alkoxide with another alkoxide. Silicon alkoxides have to be prehydrolyzed before Si T M bonds can be formed. 

The low-frequency Sn shift observed in all cases indicates a coordination expansion of the tin atom of the grafted Cll-SnCls catalyst from a four-coordinate state in its pure, unaltered form, to five- and/or six-coordination in the presence of ester and/or alcohol, from which it can be deduced that both the ester and alcohol coordinate the tin atom, which brings them in close mutual vicinity, favoring the nucleophilic attack of the oxygen of the alcohol onto the carbonyl carbon atom of the ester needed for the transesterification. 

Transition metal alkoxides are much more reactive toward hydrolysis and condensation than silicon alkoxides. This arises mainly from the larger size and lower electronegativity of transition metal elements. Coordination expansion becomes a key parameter that controls the molecular structure and chemical reactivity of these alkoxides. Hydrolysis and condensation rates of silicon alkoxides must be increased by acid or base catalysis, whereas they must be carefully controlled for the other metal alkoxides. The chemical modification of transition metal alkoxides leads to the development of a new molecular engineering. The chemical design of these new precursors allows the sol-gel synthesis of shaped materials in the form of fine powders, fibers, or films. 

Sharing alkoxy groups is the easiest way for metal alkoxides to increase the coordination of the metal atom without changing their stoichiometry. In pure alkoxides, coordination expansion currently occurs via the formation of OR bridges. Therefore oligomeric as well as monomeric molecular precursors can be found. Oligomerization depends on physical parameters (concentration and temperature) and chemical factors (solvent and oxidation state of the metal atom or steric hindrance of alkoxide groups) [7]. 

Metal alkoxides are not miscible with water, so that sol-gel reactions must be performed in the presence of a common solvent, such as an alcohol. Coordination expansion can then also occur via solvation. Solvate formation is often... 

Because coordination expansion could occur either via alkoxide bridging or solvation, the molecular complexity of metal alkoxides can be tailored by an appropriate choice of solvent. [Zr(OnPr)4] oligomers are formed (n < 4) in nonpolar solvents, such as cyclohexane, allowing slow hydrolysis rates and the formation of clear gels. Less condensed solvates are formed in propanol ( = 2) hydrolysis becomes much faster and leads to precipitation [14],... 

In covalent alkoxides M(OR)z metal atoms usually exhibit a lower coordination than in the oxide MOz/2- Coordination expansion is therefore a general tendency of the sol-gel chemistry of metal alkoxides. It occurs via nucleophilic addition, leading to the formation of oligomers, heterometallic alkoxides or oxoalkoxides. ... 

The molecular complexity of metal alkoxides also depends on the steric hindrance of alkoxy groups. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization. Trimeric species [Ti(OEt)4]3 have been evidenced in pure liquid titanium ethoxide (Fig. lb) whereas titanium iso-propoxide Ti(OPr )4 remains monomeric (Fig.la). This is no more the case for zirconium iso-propoxide which is dimeric because of the larger size of Zr(rV). Moreover solvent molecules can also be used for coordination expansion leading to solvated dimers [Zr(OPri)4(Pr OH)]2 when the alkoxide is dissolved in its parent alcohol (Fig.lc). 

An overall classification of CT adducts is not such an easy task because the formation of any heteronuclear bond actually involves some form of CT, which will evolve in different ways according to the type of bond (a) in ionic bonds, a complete CT occurs (electron transfer) (b) in heteronuclear covalent bonds, a partial CT occurs until atomic electronegativities are fully equalized (c) in dative or coordination bonds an initial CT is followed by covalent bond formation with donor coordination expansion, as in the series HOCl, HOCIO, HOCIO2, and HOCIO3 (d) in dative or coordination bonds between Lewis acids and bases the so-called Lewis adducts are formed (e.g., in the H3N + BF3 H3N-BF3 reaction) with donor/acceptor coordination expansion (e) in outer [D,A] CT adducts a simple intermolecular attraction occurs without the formation of a new chemical bond. 

Coordination Expansion Silicon can undergo 3sp d and 3sp d hybridizations, and thus the coordination of the silicon can expand to penta- and hexacoordina-tion, which lowers the activation energy for substitution reactions and ligand exchange. Transitions involving hexacoordinated and pentacoordinated intermediates are common ways to substitute silicon ligands, pathways that are less obtainable for carbon atoms, whose smaller size and exclusive 4-coordination restrict snch snbstitntion reactions. The hydrolysis, condensation and dissolution of silicate, which are described in the next section, exemplify the importance of this attribute. 

The condensation step also follows an Sf.,2 mechanism enabled by coordination expansion, but in this case, the nucleophilic attack is conducted by deprotonated... 

As discussed earlier, Ti may be coordinatively unsaturated, allowing coordination expansion via olation, oxolation, and alkoxy bridging which also contribute to enhanced hydrolysis... 

The +4 oxidation state (z = 4) is the only important one in the chemistry of silicon in naturally occurring systems [10], and the coordination number of silicon, N, is most often four. Compared to transition metals discussed in the previous chapter, silicon is generally less electropositive, e.g., the partial positive charge on silicon nucleophilic attack, and since N = z, coordination expansion does not spontaneously occur with nucleophilic reagents. These factors make the kinetics of hydrolysis and condensation considerably slower than observed in transition metal systems or in Group III systems. 

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